ABSAEOKITE. 331 



crystals of the groundmass in all directions, but do not penetrate the pheno- 

 crysts of olivine and augite, indicating that the formation of the apatite was 

 not among the earliest of the crystallizations, but took place when the 

 groundmass crystallized. 1 



The development of orthoclase and leucite from a magma so low in 

 alkalies and with so little potash is notable. The almost total absence of 

 lime-soda feldspar is plainly due to the low percentage of alumina, which 

 was nearly all combined with the alkalies to form alkali-feldspathic min- 

 erals. It is evident that both the orthoclase and leucite must be rich in 

 soda. The formation of leucite was undoubtedly controlled by the low 

 percentage of silica, which if higher would have formed a polysilicate of 

 all the aluminum and alkalies, instead of a metasilicate and polysilicate. 

 The earliest compounds to crystallize — olivine and augite — consumed the 

 magnesia and lime, with iron oxide and some alumina, and possibly some 

 soda, It is evident that the remaining magma contained lime in sufficient 

 amount to satisfy the phosphoric oxide, besides a little that combined with 

 alumina and silica to form labradorite. Moreover, the alkalies and alumina 

 were left in the proper proportions to form alkali-feldspathic minerals. In 

 a molten condition they did not exist as molecules of these minerals, yet 

 we see the indication of an influence that controlled the proportions of the 

 partially dissociated elements. At the time of the crystallization of the 

 alkali-feldspathic constituents there were small amounts of ferromagnesian 

 silicates still liquid, which crystallized at this time. 



The rock most closely related to the one just described in chemical 

 composition occurs as a dike, 4 feet wide, on the divide between Lamar 

 River and Crandall Creek, south of Hoodoo Mountain. It is dark colored 

 (1306, 1307) and aphauitic, with abundant large crystals of augite 5 to 10 

 mm. in diameter and smaller phenocrysts of olivine. On the sides of the 

 dike a thin layer of the rock is glassy and black. 



In thin section the body of the dike is holocrystalline and very fine 

 grained. The groundmass consists of indistinctly outlined lath-shaped feld- 

 spars with low angles of extinction, besides an indistinct feldspathic mineral 

 as cement, which is cloudy. The lath-shaped feldspars appear to be, in part 

 at least, orthoclase with minute lath-shaped cores of lime-soda feldspars. 



1 Arnold Hague, Notes on the occurrence of ii leucite rock in the Absaroka Range, Wyoming 

 Territory: Am. Jour. Sci., 3d series, Vol. XXX VIII, July, 1889, pp. 43-47. 



